1. Field of the Invention
This invention relates to analog-to-digital (A-D) converters, digital-to-analog (D-A) converters, and digital modulators, especially to those employing sigma-delta converters.
2. Background Information
Sigma-delta A-D converters include quantizing means for producing a digital output, oversampled relative to the signal bandwidth, and a feedback loop for feeding back a signal derived from the digital output to be combined with the analog input for input to filter means, the output of the filter means being connected to the quantizing means. This permits an improved signal-to-noise ratio to be achieved from coarse quantization by shaping the quantizing noise spectrum to suppress it within a desired bandwidth. A following digital filter removes the out-of-band quantizing noise and also reduces the sampling rate to close to twice the signal passband.
The extent to which the quantizing noise is shaped in the signal passband depends upon the order of the loop filter within the sigma-delta A-D converter. Baseband converters using single bit quantization typically employ second or third order loop filtering and bandpass converters second order filtering. The use of higher order loop filters imposes greater demands to ensure loop stability.
MASH baseband converters have been proposed (EP-A-0,368,610, EP-A-0,328,318, and EP-A-0,308,194) to provide the reduced quantizatizing noise associated with higher order loop filtering without the disadvantages of susceptibility to oscillation, by the use of two or more first or second order sigma delta converters. In these arrangements, an error signal representing the difference between the input and the output of the quantizing means of a first sigma delta converter is itself converted to digital form in a second sigma delta converter and then subtracted from the digital output of the first sigma delta converter. This error signal contains an integration of the actual error between the input and output of the first sigma delta converter, requiring a digital differentiation of the output of the second sigma delta converter before it can be subtracted from the digital output of the first sigma delta converter.
Another proposal (GB-A-2,202,100) for reducing the quantization error, but related to A-D converters in general, entails a feedforward arrangement which combines the digital output of an analog-to-digital converter with an error signal, converted to digital form, and representative of the difference between the digital output reconverted to analog form and the analog input itself.